Vehicle mounted aerial devices are widely used to perform work at elevated locations such as on utility poles and overhead utility lines. The aerial device includes a boom which is mounted for rotation in a vehicle bed and which carries on its upper end a platform that is typically a bucket or basket. Multiple section booms are common, and they may be articulating or telescoping booms. Articulating boom assemblies including a lower boom having a knuckle joint connection with an upper boom which carries the platform. The booms can both be pivoted up and down by hydraulic cylinders in order to properly position workers carried in the bucket. Telescoping boom assemblies have boom sections that telescopically extend and retract to position the platform as desired.
In both types of machines, it is necessary to maintain the floor of the platform in a level horizontal position as the boom pivots up and down. Various types of platform leveling systems have been developed to perform the leveling function. Existing leveling systems are for the most part either mechanical systems employing mechanical linkages or hydraulic systems making use of hydraulic cylinders in a master/slave relationship.
Prevalent mechanical leveling systems include parallelogram linkage systems and chain or cable systems. In the parallelogram linkage system, a link is pinned to the platform and to the turntable or base. The link is parallel to the boom but is pinned at locations offset from the boom pivot connections to the turntable and platform. The pin locations and the link and boom form a parallelogram geometry that keeps the platform floor horizontal as the boom is raised and lowered. While this type of arrangement is simple and functions well at small boom angles, it has a significant disadvantage in that the boom articulation is limited. When the boom is pivoted beyond 90.degree. (straight up), the boom and link are subjected to geometrically increasing column loads, and large stresses are applied to the pivot pins. The pivot points on the platform pass one another at some position beyond 90.degree., and the system then collapses.
The chain or cable systems are commonly used when the boom assembly is an articulating structure having two or more booms. A chain or cable is extended through the booms in a continuous loop. One end of the chain or cable is fixed to the turntable. The chain or cable extends within the lower boom and is drawn around a sheave or sprocket located at the pinned connection between the upper and lower booms. The chain or cable then extends within the upper boom and around a sheave or sprocket fixed to the platform pin on which the platform is carried. The chain or cable is then extended back down around the sheave or sprocket located between the two booms and is extended through the lower boom and secured to the turntable. The continuous loop that is made by the chain or cable provides a mechanism that keeps the platform floor in the same plane as the turntable as the booms are raised or lowered.
Although this system is also relatively simple, it is subject to several mechanical problems. The chain or cable can stretch under load, as can sections of booms that are insulated. Stretching of the cable or chain can create a situation where the platform "sags under load" and creates a "spongy" feel. The components are largely housed within the booms where access for maintenance is difficult. Also, load safety factor considerations require large boom cross-sections which increase the cost and weight of the machine. Cable systems are also subject to required mandatory scheduled replacement of parts and other significant maintenance requirements. If the platform leveling loads are large, the cables, chains, insulating rods, sheaves and sprockets must be unduly large to handle the loads.
Master/slave hydraulic leveling systems are used extensively on telescopic aerial devices. This type of leveling system makes use of two hydraulic cylinders, one of which serves as a master cylinder and the other of which serves as a slave cylinder. The master cylinder is connected at one end to the turntable and at the other end to the boom which pivots on the turntable. The slave cylinder is connected at one end to the tip portion of the boom assembly and at the other end to the platform. The two cylinders have fluid chambers that are equal in volume. The cylinders are hydraulically connected such that extension or retraction of the master cylinder, as occurs when the boom pivots up or down, causes the slave cylinder to extend or retract the same distance but in the opposite direction as the master cylinder. The geometry and the hydraulic circuitry maintains the platform floor in the same plane as the turntable as the boom is raised or lowered.
Hydraulic systems of this type are satisfactory when the angle of boom articulation is 120.degree. or less. However, this limitation in the allowable articulation is a drawback in many applications. Also, the closed hydraulic loop is difficult to purge and difficult to maintain free of air infiltration which can cause operating problems.
All of these systems are characterized by components which are located at both ends of the boom and throughout the entire boom assembly. This results in assembly and maintenance difficulties and occupies space that can otherwise be used for other purposes.